a) Field of the Invention
The invention is directed to a device and to a process for determining position between two parts which are movable relative to one another, particularly for measuring the relative rotation or displacement of parts of instruments, machines and workpieces. The device is intended particularly for angle measurement in geodetic instruments.
b) Description of the Related Art
A large number of solutions for angle measurement are known from patents, publications and commercially available instruments. Systems in which a light-dark division on a glass carrier is provided as material measure have found widespread use, this light-dark division being imaged on or sensed by an optoelectronic sensor. The measurement signals obtained in this way are evaluated by means of a subsequent evaluating unit to obtain measurement values. There are systems with one or more parallel graduation tracks, systems which measure incremental or absolute values, and linear or surface sensor arrangements.
For user-related reasons and for cost reasons, systems with a linear sensor arrangement measuring absolute quantities have gained increasingly popularity. With a large amount of available information, the use of a standard CCD line, for example, offers an enormous advantage with respect to cost and availability compared with customized sensor arrangements. The rough value information is serially encoded in a code track and the refined or precise value information is obtained through interpolation of the rough value information.
In DE-OS 4 436 784, a scale with a code is realized in a binary manner and by means of equidistant bars. This graduation can also be read incrementally.
In DE-OS 196 38 912, a scale with a code is realized by a pseudostochastic code. The illumination device is pulsed to enable measurements with rapid movement.
WO 84/01027 describes another system with pseudostochastic code upon which a Manchester code is superimposed for increasing resolution.
In EP 0 085 951, the interpolation between equidistant bars of different widths is refined by multiple digital determination of the light centroids of every light bar with different threshold values followed by averaging.
DE 2 619 494 describes an absolute measurement system with serial binary coding of the scale and readout of the latter by linear CCD sensor lines. The resolution amounts to the width of a sensor element and is increased by optical magnification of the graduation scale.
The evaluation of the rough coding of a pseudostochastic graduation requires either a high computing output with generation of the comparison code in real time or a high storage requirement with comparison with the completely stored code. All known methods make use of only a small number of light-dark transitions and dark-light transitions (flanks) to obtain the interpolation value. Therefore, in order to realize a high resolution every flank must either be quantized with an A-D converter at an increased expenditure on hardware or must be digitized by means of a plurality of different threshold values (increased measurement time). Due to the small number of flanks, graduation errors and contamination of the graduation have a great impact on measurement accuracy.
Therefore, it is the primary object of the invention to provide a device and a process for position determination between two parts which are movable relative to one another in which the disadvantages of the prior art are eliminated and in which highly reliable measurements, an accuracy in the arc second range and the possibility of modularity are achieved with very low expenditure on optics and electronics and with simple assembly and adjustment.
According to the invention, this object is met in a device for determining position between two parts which are movable relative to one another, and which comprises a linear sensor arrangement, a length scale or angle scale comprising a sequence of transparent and non-transparent fields, an illumination device and elements for imaging at least one section of the scale on the sensor arrangement. The entire length scale or angle scale is divided into n graduation portions of equal length, wherein every portion contains a sequence of transparent and non-transparent marks, and the sequence is successively a number which is binary-coded with k bits and correlated with the respective portion as rough value code and a fixedly recurring number which is likewise binary-coded with k bits as a reference mark (reference mark with length of k bits). Every code bit is of equal length and is formed of a transparent and a non-transparent field (half-bit) of equal length, wherein the binary information comes from a sequence of two half-bits. A process for relative position determination shows how the sensing of the graduation and the evaluation and determination of the measurement values is carried out.
It is advantageous when the reference mark having a length of k bits in the device encodes in binary code a number which occurs at no other section of the graduation with a length of k bits other than at the reference mark itself.
Further, for obtaining measurement values it has proven advantageous that the code graduation is divided into 250 numbered portions of equal length, the numbers of the portions and the reference mark are each encoded with 8 bits, the reference mark has the value 254, and the 250 portions are coded with numbers 4 to 127, 129 to 253 and 255, wherein the numbers 0 to 3 and 128 are not coded for portions. Thus, portions 0 to 3 are cut out and the numbering first starts at 4.
A simple construction of a device according to the invention results when the imaging of the graduation portion on the sensor arrangement is realized by pure projection without the use of further optical elements by means of a divergent illumination source of small apparent diameter. For this purpose, a virtually punctiform light source, e.g., a LED, is advantageously used for the illumination of the scale.
The transparent and non-transparent fields of the length scale or angle scale are whole-number multiples of a smallest bar width h.
In implementing the process, it is advantageous when the linear magnification or imaging scale a is selected in such a way that, with a uniform relative displacement between the code graduation and sensor line by the length of a sensor element, the quantity of the changes in the correlation of light-dark transitions and dark-light transitions to the sensor elements increases approximately uniformly from 0 to the flank number in the evaluated graduation portion.
The imaging scale a is the quotient of the pixel width and the width of the image of a half-bit on the sensor line of the sensor arrangement, namely,
a=pixel width/width of the image of a half-bit on the sensor line.
A simple procedure in imaging the portions of the code graduation on the sensor arrangement results when the imaging is carried out without additional optical components. The imaging can accordingly be carried out by means of a light source emitting a virtually punctiform divergent light, e.g., a suitable LED, with zenithal or central projection on the sensor arrangement arranged close behind the code graduation in the direction of light.
In order to increase measurement accuracy, it is advantageous when a plurality of measurements are carried out making use of the signal noise and the measurement results are averaged. A further advantageous possibility for increasing measurement accuracy consists in that a plurality of measurements are carried out with different trigger thresholds and the measurement results are then averaged.
Accordingly, it can also be advantageous for a further increase in measurement accuracy using an integrating optical sensor line when a plurality of measurements are carried out with different exposure times and the measurement results are averaged.
The invention will be described more fully in the following with reference to an embodiment example.